As is commonly known, the stepping motor is generally employed in open loop because it has two specific characters that (1) its running speed is proportional to a generating speed of the pulses and (2) its revolutionary frequency is proportioned to a generating number of the pulses. However, if the stepping motor is used in open loop, there may be some cases where it runs into unforeseen disorder due to a fluctuation of the load falling on the motor. For this reason, when a stability of the performance is specifically requested, a closed loop control system is generally used which is possible to change an excitation mode on the basis of informations showing a current phase of the stepping motor which are obtained by feeding encoder output signals back to a pulse distributor.
Where using the stepping motor in the closed loop in this way, the aforementioned disorder due to said load fluctuation can positively be prevented, however, it is impossible to give instructions of intermediate speed and in consequence to set freely the speed of stepping motor because such a speed control method is to be realized by changing over an excitation mode prepared in advance.